1. Field
Disclosed herein is a method for producing a current metering device which can be used for example in electricity meters or power meters.
2. Description of Related Art
For current metering or power metering, various electronic electricity meters are known which are increasingly replacing mechanical Ferraris meters in industry and the household and which meter current with arrangements of different mechanical and electrical structure. In addition to current metering with measurement shunts, Rogowski coils or Hall elements, current transformers based on soft magnetic annular cores, especially annular band cores, as magnetic modules, are common in electricity meters, A magnetic module (current transformer, transformer) causes electricity mains isolation and delivers a precise measurement quantity in the form of a signal voltage to a load resistor, The demands for amplitude accuracy, phase accuracy, and linearity are established by IEC 62053, −21, −23 or previously 1036 in Europe and ANSI C12.xx in the USA and can be found for example in the company brochure “VAC-current transformers for electronic power meters”, of Vacuumschmelze, October 1998, Embodiments of current transformers for electronic power meters are generally also known from the company brochure “Current transformers for electronic power meters” of Vacuumschmelze 2002, Power meters using these current transformers (also called watthour meters) are used as officially approved measurement means to bill the electrical current which represents the power consumption and which is used by a consumer for the utility.
A structure of busbars which form so-called primary conductors and an annular core current transformer which matches them for metering of current consumption is conventional, Plug-in electricity meters which are common in the USA and other countries have standardized rectangular terminal lugs on the back which are pushed into slots with suitable spring contacts when the electricity meter is mounted, These terminals with a cross section of roughly a×2.5 mm are used for feed and discharge of the current consumed which is a maximum of roughly 200-480 Aeff in 110 V systems. The width a of the cross section is for example a=19 mm at a maximum current of Imax=320Aeff. Conventionally the currents of the three phases of the AC network are routed into the electricity meter, through a current metering system and again out of the electricity meter.
The current transformer can be made such that a busbar measuring for example 19×2.5 mm can be inserted through an inner hole of the current transformer. The region of the busbar on which the current transformer is to be located can also have a round cross section so that the inner hole of the current transformer can be made smaller and accordingly a smaller and more economical annular band core can be used. Even if the production time of the core and the winding time are otherwise the same, the consumption of high quality magnet material and the process steps of heat treatment and coating are more favorable when the diameter of the core is smaller. A busbar suitable for this purpose is produced by making available a U-shaped conductor arrangement with different sections, A central connecting section with a round cross section is used as an element of the current conductor for routing through the corresponding opening in the current transformer. Two terminal conductor sections with a rectangular cross section are used to connect the current conductor in the form of the conventional plug-and-socket connections explained above.
When the current transformer is mounted on a one-piece primary conductor, at this point it is critical to slip the inductive transformer on the primary conductor together with its terminal contacts. Thus the minimum inside diameter of the magnetic transformer is necessarily determined by the size of the plug-in contact in a primary conductor produced from one piece.
If the primary conductor is made of several individual parts, it is possible to adapt the inside diameter of the inductive current transformer to the minimum which is possible from the electromagnetic design, however increased cost in the assembly of the primary busbar must then be tolerated. The conductor arrangement consists of three metal parts with differing cross sections, and the two ends of the round current conductor can be attached to the flattened surfaces of the rectangular terminal conductors. Conventional joining methods in producing busbars are brazing and welding methods, In both methods it is critical to protect the current transformer from the heat which arises in the joining process, for which complex structures with cooling tongs between the joining site and current transformer are necessary.
Another disadvantage of this method is the very limited possibilities of process monitoring of the joining method. Reliable monitoring of the connecting site is essentially only possible by destructive testing. To circumvent these disadvantages of thermal joining methods, for example DE 10 2004 058 452 has proposed carrying out the joining process in the form of cold pressure welding. In this method the action of heat during the joining process is avoided, but the resulting connections of the individual parts of the primary conductor have other defects. Thus only a fraction of the connecting surface consists of cold pressure welded material. Most of the connecting surface is connected only positively with the result that an air gap in the micron range remains between the joining partners. This gap reduces the current carrying capacity of the connecting point with the result of possibly unduly high heating of the joining site when the conductor is loaded with the maximum current.
The connections of this conductor arrangement of three elements with cross sections which differ from one another at the connection points should enable a long service life of for example roughly 10-15 years with high reliability so that production of the conductor arrangement must be done in a very process-reliable manner. For reasons of electrical conductivity the corresponding busbars or conductor arrangements are made primarily of copper material. But problems arise both in brazing and also welding, especially due to the heating in the preparation of the connecting points since the heat is transferred through the current conductor to the current transformer and can damage it.